The present invention relates to a new type of all-metal solid-state component referred to herein as the GMR "transpinnor". More specifically, a multilayer metal structure is described herein which employs the phenomenon of giant magnetoresistance (GMR) to function as an active device with both transistor and transformer properties. In addition, a structure is described herein which functions as a passive transformer. It is well known that there is no transformer, either passive or active, in semiconductor bipolar technology. The active devices are superior to conventional thin-film transformers in two ways: (1) they have power gain, and (2) they have flat response down to and including dc. The duality of function of these active devices, as both transformer and transistor, renders them as a truly new type of basic electronic component, which we term a "transpinnor." In addition to their basic roles as switching devices (e.g., transistors) and transformers, transpinnors are well suited to provide the foundation of general-purpose all-metal electronics, both analog and digital. They provide functionalities corresponding to a variety of electronic and magnetic circuit components. These components include differential amplifiers, memory elements, and gated and pulse transformers. According to various embodiments, a GMR transpinnor with two input leads, two output leads and two power leads is described. Several methods of achieving anhysteretic (i.e., without hysteresis) films for linear transpinnor operation are also described.
The so called all-metal spin transistor was described in articles by Mark Johnson in Science (page 320, volume 260, Apr. 16, 1993) and IEEE Spectrum (page 47, May 1994), both of which are incorporated herein by reference. The device described is a bipolar transistor, in that it relies on two different carrier types. Whereas the carriers for silicon bipolar transistors are electrons and holes which have opposite electric charge, the two carrier populations for the all-metal spin transistor both comprise electrons which have opposite spin alignments. Generically, the Johnson all-metal spin transistor is a single-period, three-layer structure in which electric current is passed from layer to layer in the direction of the film normal. The Johnson transistor utilizes the fact that the lowest energy-conduction band in a ferromagnetic metal is for electrons with spin polarization in the direction of the magnetization, and the lowest energy state in a nonferromagnetic conductor is for equal populations of spin polarizations. The emitter and collector layers of the Johnson spin transistor are ferromagnetic films, and the base layer is a nonmagnetic metal. The output of the device is adjusted by changing the angle between the two magnetizations, i.e., by switching the magnetization direction of one of the two films so the relative orientations of the respective magnetization directions change between parallel and antiparallel alignments.
An all-metal spin transistor has several potential advantages for high-density applications. For example, because submicron lithographic techniques can readily be applied to its fabrication, it is expected that the all-metal spin transistor can be made qualitatively smaller than semiconductor bipolar transistors; possibly even 100 times as dense. Moreover, because an all-metal transistor is exclusively metal, it exhibits much greater carrier density than highly doped silicon. High carrier density will enable the spin transistor to operate with much smaller feature sizes than silicon transistors. In addition, the switching time of the spin transistor is projected at 2 ns or better.
Finally, because the spin transistor is an all-metal device, its fabrication will not require many of the high-temperature process steps inherent in the fabrication of silicon devices. This becomes even more significant when viewed in the context of a new all-metal GMR memory element described in commonly assigned U.S. Pat. No. 5,587,943 for NONVOLATILE MAGNETORESISTIVE MEMORY WITH FULLY CLOSED-FLUX STRUCTURE issued on Dec. 24, 1996, the entire specification of which is incorporated herein by reference. According to specific embodiments of the invention described in that commonly assigned patent, the all-metal GMR memory element may be employed in a random access memory array, hereinafter referred to as a permanent random access memory (PRAM). Given that the memory elements of the PRAM are all-metal devices, it becomes apparent that it is at least theoretically possible to construct an all-metal random access memory using the all-metal spin transistor as the basic building block for the device's selection electronics (e.g., word and digit drivers, selection matrices, low-level sense gates, differential sense amplifiers, etc.). In fact, the referenced application briefly discusses this possibility. Not only would such a device have the speed and density advantages described above, it would entirely eliminate the need for any semiconductor processing steps in its fabrication.
However, despite the numerous potential advantages of the Johnson spin transistor, its implementation in various devices presents a practical problem because of the low operating range of the absolute value of its impedance. The difference in impedance between the "on" and "off" state of a Johnson spin transistor is only a few microohms. Moreover, the difference in output voltage between the maximum and minimum voltages is only a few microvolts (see Johnson, referenced above). These differences are too small to be useful in most applications. Indeed an "off" impedance of only a few microohms is a close approximation to a dead short.
From the foregoing it is apparent that an all-metal switching device with "on" and "off" resistances more closely matched to the characteristics of the GMR memory elements described in U.S. Pat. No. 5,587,943 is desirable.